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How to select the appropriate solar collector system?

With methods of determining the dimensions and calculator!

Author: Mónika Vértesy
2011-06-22


Which properties of a plain and and a vacuum tube collector determine the beneficial application? Which are the pitfalls of supplemental heating with solar collectors? For the most typical use, the production of DHW, we already have a calculator, so everybody is able to calculate their necessary collector surface!



The collector should be plain or vacuum tube?

To make the right decision, it is necessary to know our purpose with the collector.

1. efficiency

In the interval of the DHW temperature, the vacuum tube collectors have a slightly better efficiency annually. Why is that?

Due to the cylnder shape of vacuum tubes, the sunlight reaches the surface of the glass vertically almost the whole day, so the heat output is good from the morning until late afternoon. The plain collector reaches its peak of efficiency when the sun is positioned high, so generally during the summer around midday. On the other hand the vacuum blocks the loss of heat from the inside of the tube, this way the wind and cold temperatures have a smaller effect on the vacuum tube collector, not to mention that it also utilizes the filtered light with greater efficiency. To sum up, plain collectors are better for summer use, while vacuum tube collectors are optimal for the winter.

2. installation and orientation

The installation of vacuum tube collectors is easier, but it is also true that the tubes are more vulnerable and have a shorter life than plain collectors. This comes mainly from the fact that the chance of the vacuum escaping is quite real. However, the vacuum tube collectors have that good quality, that the tubes can be rotated, so the disadvatageous orientation can be slightly corrigated. For an articulated roof surface the plain collector can be the solution, as these are avaible in smaller sizes too.

3. expenditures

After the pros and cons mentioned above, it is time for the most important factor: vacuum tube collectors cost more than twice as much as plain collectors with the same performance.

DHW production with solar collectors

The most common field of use of sollar collectors is DHW production. The necessity of a person of DHW is about 50 litres daily, which requires a performance of 900 kWh to make annually. This would mean and expenditure of 43000 HUF (145€) if we produced it only with electricity. It worth covering about 60% of the needs annually, not more, becouse we would overproduce the necessary amount greatly in the summer, while in December and January, only about 20% of the needed heat amount would be produced.

vacuum tube collectors

vacuum tube collectors

Solar collector calculator: how large surface do you need?

Calculate yourself the size of collectors you need, try the vacuum tube solar collector calculator! In an optimal situation the collectors are installed in a 45° angle, orientated to the south. Clearly the situaton and the properties of the building not always allow this. The calculator also shows the effect of the angle and the orientation on the efficiency.

Why we do not recommend solar collectors for supplemental heating?

The first and most obvious response: the sunlight is the least intense when we want to turn on the heating: in winter. Of course, just becouse of this one reason, technically it is not an impossible idea to have collectors supporting a low-temperature radiant heating system, but this will be economical only if we can use the produced heat in the summer for other goals than producing DHW, for example pool heating (see the next paragraph). The same time, it is quite easy to get in a situation where we are not able to utilize the extended collector surface of supplemental heating, so in this period of the year we are obliged to cover up a part of the collectors or drain the water-glycol mix. The second one is an absolute waste, as the circulating liquid should be changed only once every five years, not annually.

Pool heating

We recommend vacuum tube collectors for interior, all-year-long operational pools and plain collectors for exterior pools used only during the summer. The reflection of vacuum tube collectors is stronger compared to plain collectors, so their efficiency is worse during the summer, also, the exterior temperature and the sunlight are also factors in the temperature of an exterior pool. However, the mentioned factors do not have a significant effect on interior pools, not to mention that they are used in winter as well as in summer, so the advantages of vacuum tube collectors are getting highlighted.

solar system circuit diagram

solar system circuit diagram

Solar Keymark classification

Before buying solar collectors, it worth taking a look at the page of ESTIF ( European Solar Thermal Industry Federation) where we can check wether the chosen model has a Solar Keymark classification. The brand – which can be requested voluntarily with any product of heat producing systems using solar collectors – signs that the marked product fulfills the requirements of the related European standards. This is a classification approved in a wide range and has a high standard, becouse in this case, it is not the manufacturer who provides data, but the classificating institute makes its investigations about those details which seem important to them. This could even mean investigations at certain phases of the manufacturing process, eliminating the back-stair opportunities lying in one-sided communication.

If not only solar collectors, but a whole solar system is wanted

Customers like complex solutions, as it is not necessary to find out the „ingredients” by themselves, so the chance of appearing any unseen expenditure or making mistakes is smaller. This is why nowadays the manufacturers offer complex systems to supply the DHW need of family houses. These systems contain the hydraulic block with a storage unit, an expansion tank, a pump, the safety-, mix-, and change-valves, the collection of fittings and the antifreeze liquid besides the solar collectors. In this case, the designer/client only have to answer the following questions:

- the system will operate with the boiler of the manufacturer or with a boiler of other manufacturers?

After the questions we will present the marking system of Ariston, this can be useful to orientate well. The „Kairos CF” expression is the name itself of the plain collector.

In case of Ariston boiler: Kairos Premium CF 2 200/2 TT

In case of extranaeous boiler: Kairos Compact CF 2 200/2 TT

- the boiler will be combi or simple?

In case of simple boiler: Kairos Premium CF 2 200/2 TT

In case of combi boiler: Kairos CF 1 200/2 TT

- will the collectors be installed on gable or flat roof, or on the ground?

In case of installing them on a gable roof: Kairos Premium CF 2 200/2 TT

In case of instaling then on a flat roof or on the ground: Kairos Premium CF 2 200/2 TR

- how may people will be served by the system?

This determines the size of the storage unit and the number of collectors.

example for 4 people: storage unit of 200 litres and 2 collectors: Kairos Premium CF 2 200/2 TR

example for 7 people: storage unit of 300 litres and 3 collectors: Kairos Premium CF 2 300/3 TR

An example of calculations

For those with an enterpreneurship, it is advised to create their own system. Now we dare to do it, or at least, we present the process of the calculations!

We assume having a family of five people and they need a storage unit of 300 litres. (four people usually can be served by storage unit with 200 litres of capacity, but if we are more, or there is a baby, a young children, it is worth choosing a larger storage unit, say, with a capacity of 300 litres.)

1. Choosing the collectors: using the calculator and selecting the ideal orientation and a cover proportion of 60%, two collectors with the surface of 2m2 seem to be the ideal choice. The total length of the tubes, which are connecting the boiler with the solar units is, say, 10 metres in case of a family house with one storey.

2. Determining the pressure loss of the collectors

The ideal rate of flow in case of certain collectors is a data of catalogues, in our case, let it be 60l/h (it is important no to calculate with minimal or maximal rates of flow). So the total rate of flow of the two collectors is 120l/h, which means 19mbar pressure loss. (chart)

3. Determinig the diameter of the tube

In case of larger systems, the appropriate diameter is chosen based on the rate of flow. In case of smaller systems, the situation is even more simple, we always use a tube with a diameter of 18 mm, which has a pressure loss of 2,32 mbar/m.

4. Determining the pressure loss of the system, choosing the pump

- pressure loss of the collectors: 19 mbar

- pressure loss of the heat exchanger of solar storage unit: 20 mbar

- pressure loss of the tubes: 10m * 2,32mnbar/m = 23,2 mbar

The total pressure loss of the system is 62,6 mbar. This is important, as a pump is needed be chosen with pressure producing capability enabling it to exceed this value at given water transport.

5. Selecting the expansion tank

Expansion tanks are a critical point of solar collector systems. Its purpose is to level the volume change caused by temperature changes in the closed collector circle filled up with antifreeze liquid.

Expansion tank

expansion tank

This is important when the storage unit is already warmed up to the operating temperature, but the Sun is keeping on shining intensively. We, on the other hand, are on vacations so there is no DHW consumption. For the appropriate calculation it is necessary to know the total volume of the system.

Volume of collectors: 2*5,7 l

Volume of heat exchanger: 9,6 l

Volume of hydraulic unit: 1 l (approx.)

Volume of the liquid circulating in the tubes: 2 l

This is 24 litre in total. Applying the most simple calculation method (when volume of the whole system have to be multiplied by 0,09 to get the required size of the expansion tank) we get a tank size of 2,16 litres. It is obvious that the one with the smallest size which exceeds this value has to be chosen from the supply.

In the field of calculations, we developped our skills at the education organized by Ariston, thanks for the useful help.

Related articles:

What kind of heating is the best in a passive house? Part 1

Curiosities from the world of heat recovery ventillation

Back to the articles of construction

Comments:

  • 1 - 2018. jan. 13 14:23:32

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